As an increasing number of applications and services are being made available over networks such as the Internet, an increasing number of content, application, and/or service providers are turning to technologies such as cloud computing. Cloud computing, in general, is an approach to providing access to electronic resources through services, such as Web services, where the hardware and/or software used to support those services is dynamically scalable to meet the needs of the services at any given time. A user or customer typically will rent, lease, or otherwise pay for access to resources through the cloud, and thus does not have to purchase and maintain the hardware and/or software needed.
In this context, many cloud computing providers utilize virtualization to allow multiple users to share the underlying hardware and/or software resources. Virtualization can allow computing servers, storage device or other resources to be partitioned into multiple isolated instances (i.e. virtual machines) that are associated with (e.g., owned by) a particular user (e.g., customer). Conventionally, each virtual machine running on a host computing device is assigned a portion of memory (e.g., random access memory), processing capacity and/or other resources available on that device. However, some virtual machines frequently remain idle over relatively long time intervals, and only need access to their respective resources during short periods of time. During such idle time intervals, the resources which have been allocated to those virtual machines are generally not being utilized. In some cases, it would be desirable to utilize such unused resource capacity in some efficient way, without severely impacting the virtual machines during the times when they do need access to the resources.